1. Field of the Invention
The present invention relates to a gas sensor including a sensor element for determining, for example, the concentration of a particular component of a gas which is to be measured (hereinafter also referred to as a “gas-to-be-measured”), and also including a heat-generating element for heating the sensor element.
2. Description of the Related Art
A hitherto known gas sensor includes a detection element which generates an electromotive force which varies in association with the concentration of a particular gas component (e.g., oxygen) contained in the exhaust gas of an automobile, etc.
For example, as described in Patent Document 1, such a known gas sensor includes a sensor element incorporating a detection element having a pair of electrodes (sensing electrode and reference electrode) which sandwich a solid electrolyte body, and a heater member having an insulating substrate incorporating a heat-generating element for heating the detection element, the heater member being stacked on the detection element.
Meanwhile, as described in Patent Document 2, a gas sensor has been known including an oxygen pump cell and an oxygen concentration determination cell. In this gas sensor, the magnitude and direction of current flowing between a pair of electrodes of the oxygen pump cell are controlled so that the electromotive force generated between a pair of electrodes (sensing electrode and reference electrode) of the oxygen concentration determination cell is a reference voltage, whereby oxygen is pumped out of or into a gas detection chamber (measurement chamber). Thus, on the basis of the current flowing through the oxygen pump cell, the concentration of oxygen contained in exhaust gas is determined, along with the air-fuel ratio of the exhaust gas.
In the aforementioned conventional techniques, each of the sensing electrode, the reference electrode, and the heat-generating element has a conduction portion (lead) which is provided along the surface of a solid electrolyte body or an insulating substrate so as to achieve electrical conduction to, for example, a circuit. Generally, such a lead is formed of the same material (e.g., noble metal) as the sensing electrode, the reference electrode, and the heat-generating element.
For forming the reference electrode or the heat-generating element, a material containing noble metal powder having a large particle size (e.g., Pt powder having a mean particle size of 1.5 μm or more) has conventionally been employed. However, when the lead is formed of a similar material, the amount of a noble metal employed is increased for reducing resistance, since the lead is required to have higher electrical conductivity.
In order to cope with such a problem, the amount of a noble metal employed in the lead is reduced by forming the lead from a material containing a noble metal powder having a small particle size (e.g., Pt powder having a mean particle size of less than 1.5 μm).
[Patent Document 1] Japanese Patent Application Laid-Open (kokai) No. H07-55758
[Patent Document 2] Japanese Patent Application Laid-Open (kokai) No. 2007-33114
[Patent Document 3] Japanese Patent Application Laid-Open (kokai) No. 2011-117937
3. Problems to be Solved by the Invention
However, when a material containing a noble metal powder having a small particle size is employed for forming the lead as described above, sintering of the material is promoted during production of a sensor element, and the lead is formed of a sintered compact having no voids (or having a small number of voids).
Thus, when, for example, the number of voids is reduced in the reference electrode lead, the lead exhibits poor gas permeability, and thus may fail to permeate oxygen (i.e., reference source) therethrough (e.g., oxygen gas accumulated in the reference electrode may not be removed). Therefore, the reference electrode may fail to exhibit its required performance.
In the case where, for example, the number of voids is reduced in the heat-generating element lead; i.e., the heat-generating element lead exhibits poor gas permeability, when the gas pressure in the heat-generating element increases in association with temperature elevation, a reaction between the internal gas (oxygen) and a noble metal is promoted. Thus, breakage may occur in the lead, resulting in premature failure.
Meanwhile, a technique has been proposed in which porous carbon patterns are formed on the surface of a lead for increasing the amount of gas permeation (see Patent Document 3). However, this technique requires a complicated structure, along with a large number of production steps.